There's some controversy regarding the Aqualung Leg3nd MBS and why the concept of a combined Venturi lever-adjustment knob would be either interesting or just stupid.
But I don't really understand its disadvantages. Can someone give examples of where you'd really want to adjust both separately? Is it because breathing near surface would be unnecessarily stiff and with disproportional effort? Then again, why would you tighten it submerged at surface-level?
They adjust 2 different things.
The knob, in simple terms, adjusts the spring pressure on the valve that opens and closes to let air flow. Over time, particularly just after new or newly serviced, the valve seat can wear in just a bit and a finely-tuned 2nd stage can start to hiss (i.e. freeflow a very small amount). Turning that knob just a little bit to slightly increase the spring pressure on the valve seat will stop that.
The venturi lever controls a (generally) plastic vane inside the 2nd stage that directs the air coming out of the valve one way or another. In one position, it directs the air in way that causes it to swirl around inside, creating a venturi effect which has result of pulling more air out of the valve.
With the venturi lever in the other position, it directs the air straight out of the mouthpiece, producing no venturi effect.
I think a good analog is how it works if you are pouring water out of a bottle. If you invert the bottle and let the water run out, it comes out at one speed. But, if you swirl the bottle once or twice to get the water started swirling inside, then it drains out even faster.
The practical effects are two:
With the lever in the Dive position, if you are very sensitive, you can inhale and feel like more air is coming into your mouth or the air is coming in more easily. If you are not that sensitive, you won't notice a difference.
Also, if you are not underwater, and the 2nd stage is tuned correctly, you can tap the purge button on the front of the 2nd stage. If the lever is in the Pre-dive position, you will hear a short burst of air and it will stop immediately. If the lever is in the Dive position, the reg will freeflow. I.e. in the Dive position, if you tap the purge (on the surface), air will start coming out and not stop until you do something to stop it.
That is the reason to put that lever in the Pre-dive position when it's not in your mouth.
Since the knob and lever adjust two completely different things that are not necessarily related, I'm not sure why you WOULDN'T want them to be separate adjustments. If my valve seat wears a little and I need to turn the knob in some, or if I'm swimming to a current that is pushing on the purge and causing a little bit of a freeflow, so I need to adjust the knob for that, I don't want my adjustment of the knob to do anything with the venturi vane.
Side note: I don't know about the AL 2nd stage you mentioned. But, I do know that Atomic 2nd stages have a feature unique to them. They have a knob, but they don't have a venturi lever. Instead, they have what they call AFC (Automatic Flow Control, I think). They do have a venturi effect vane inside. But, instead of needing the diver to manually flip it back and forth, the AFC reacts to water pressure and adjusts the vane automatically. When you submerge and descend the increasing water pressure causes the vane inside to change to the Dive position. When you surface, the pressure goes away and the vane changes back to the Pre-dive position.
